Evaluating spatial resolution in prostate MR images: the utility of the ladder method

Evaluating prostate MRI resolution is challenging due to motion artifacts caused by body movement or intestinal gas, which can degrade image quality. One simple approach to resolution assessment is the ladder method, but reports of its application to clinical images remain limited. This study aimed to compare the ladder method with visual evaluation to assess its utility.

T2-weighted images of the prostate in healthy volunteers and a ladder phantom at various resolutions with altered pixel sizes (0.5, 0.7, 0.9 and 1.1 mm) were acquired. Three radiologists conducted visual evaluations of the prostate images. Correlation coefficients between the visual evaluation scores and the ladder index (LI) obtained from the ladder method were compared.

Average visual evaluation scores were 7.7, 5.9, 3.8, and 2.1, while the spatial frequencies corresponding to LI = 0.5 (50%LI) were 0.86, 0.81, 0.54, and 0.42 cycles/mm for each pixel size ranging from 0.5 to 1.1 mm, demonstrating higher values at smaller pixel sizes. The correlation coefficients exceeded 0.7 at most spatial frequencies, indicating a strong correlation. Inter-reader agreement was high (Kendall's W = 0.87), indicating consistent evaluation among radiologists.

Spatial resolution for prostate MRI could be evaluated objectively using the ladder method.